A. Field of the Invention
This invention relates to the use of light sources to cure polymeric materials, particularly those used in dentistry, and through that curing to tailor the post-cure properties of the materials in order to achieve desired physical properties in the materials. The invention includes various modulation schemes for causing polymeric materials to cure in a desired manner.
B. The Background Art
In the prior art, curing dental materials by the use of light was known. For example, U.S. Pat. No. 5,472,991, which is hereby incorporated by reference, discloses a dental composition that can be polymerized by the use of two curing steps using light of different wavelengths during the two steps. U.S. Pat. No. 5,350,834, which is hereby incorporated by reference, discloses photoinitiated control of inorganic network formations in the sol-gel process. U.S. Pat. No. 4,872,936, which is hereby incorporated by reference, discloses photocuring of mixtures which could be used in dentistry or medicine. Other patents to which the reader may wish to refer to understand the background against which the invention was made are U.S. Pat. Nos. 4,191,622, 4,224,525, 4,273,335, 4,298,005, 5,002,854, 5,002,855, 5,154,861, and 5,175,077, each of which is hereby incorporated by reference.
In the field of dentistry, there is a significant trend away from use of metal materials for repair and reconstruction of teeth and the construction of dental appliances. Dentists and dental technicians are now relying on polymeric materials for applications which in the past required the use of metals. Polymeric materials are preferred by many dentists due to their ease of formation, superior aesthetic results, and avoidance of concerns about release of mercury from amalgams used in other dental restorative materials.
Polymeric dental materials can be very durable if prepared and cured properly. In particular, the polymerization process that the materials undergo should be tailored to provide a hard and durable resulting dental appliance or reconstruction, but should not exhibit brittleness, stress cracking, shrinkage or other undesirable qualities. The particular application of a dental material requires that it has physical characteristics tailored to that application in order to maximize performance.
It is an object of the invention to provide a method and system for curing polymeric dental materials. It is a feature of a preferred embodiment of the invention that various lasers are used to cure polymeric dental materials. It is a consequent advantage of the invention that the dental materials may be cured quickly and to a predetermined physical state.
It is an object of the invention to provide a method and system for tailoring the post-cure properties of dental materials. It is a feature of the invention that light is applied to the dental materials to cause their polymerization and cure.
It is an object of the invention to tailor the post cure properties of dental materials. It is a feature of the invention that light is applied to the dental materials according to various power, wave form and modulation parameters in order to cause the particular dental materials to cure into a final form with desired properties. It is a consequent advantage of the invention that the performance of the dental materials may be optimized for a particular application or environment by curing techniques.
It is an object of the invention to utilize a light power source in order to cure dental materials on an intermittent or sporadic basis so that a single light power source may serially provide power to several physically discrete quantums of dental materials to be cured. It is a feature of the invention that some preferred embodiments of the invention involve applying light to a dental material in a periodic fashion such as on/off, so that while a first dental material is experiencing the off-phase of its cure, the light power source may be used to provide light and power to the on-phase of a second dental material to be cured. It is an advantage of the invention that in a dental office having several dentists or technicians, multiple dental material curing lights could be powered for simultaneous use by a single light power source, reducing the cost of the capital investment in equipment.
It is an object of the invention to affect the growth of polymer chains in polymeric dental materials. It is a feature of the invention that light source power modulation is employed in order to initiate and control the growth of polymer chains in polymeric dental materials so that the resulting materials have chains of a desired length, resulting a cured dental material with desired strength, hardness, lack of brittleness and other properties desired for its particular use.
It is an object of the invention to provide a light source power modulation scheme that minimizes shrinkage of a dental material during cure. It is a feature of the invention that light may be first applied to the dental material at high power level, dropped over time to a lower power level and then maintained at the lower power level. It is an advantage of the invention that such a modulation scheme minimizes shrinkage in a dental material.
It is an object of the invention to provide a light source power modulation scheme that creates a posture dental material that is flexible. It is a feature of the invention that light may be applied to the dental material at a first power level, held constant at that first power level for a period of time, and then increased over time to a second power level. It is an advantage of the invention that such a modulation scheme produces flexibility in a dental material.
It is an object of the invention to provide a light source power modulation scheme that creates a post-cure dental material that has great surface or wear strength. It is a feature of the invention that light may be applied to the dental material at a first high power level, quickly reduced to a lower power level, and then gradually increased to about the first high power level again. It is an advantage of the invention that such a modulation scheme produces significant surface or wear strength in a dental material.
It is an object of the invention to provide a light source power modulation scheme that creates a post-cure dental material that is useful as a pit and fissure sealant. It is a feature of the invention that light may be applied to the dental material at a first high power level, decreased to a second lower power level, and then held constant at about that second power level for a period of time. It is an advantage of the invention that such a modulation scheme produces a dental material that has the qualities needed to serve as a pit and fissure sealant.
It is an object of the invention to provide a light source power modulation scheme that creates a post-cure dental material that is useful as a bonding agent for indirect applications. It is a feature of the invention that light may be applied to the dental material in an increasing and decreasing pattern according to a sine wave, particularly initiating light exposure at a mid-power level on the sine wave. It is an advantage of the invention that such a modulation scheme produces a dental material that has characteristics suitable for use as a bonding agent for indirect applications.
It is an object of the invention to provide a light source power modulation scheme that creates a post-cure dental material that is useful as a bonding agent for direct applications (such as orthodontics). It is a feature of the invention that light may be applied to the dental material according to a discontinuous waveform. For example, the waveform may include initiating power and holding it constant for a period of time at a first low power level, turning power to the light source off for a period of time, and then reinitiating power and holding it constant for a period of time at a second higher power level. It is an advantage of the invention that such a modulation scheme produces a dental material useful as a bonding agent for orthodontic applications.
Many other modulation schemes and tailoring the post-cure characteristics of the dental material or other polymeric material for almost any use are possible within the inventive concept. Other objects, features and advantages of the invention will become apparent to persons of ordinary skill in the art upon reading the specification and reviewing the appended figures.
A. General Description
The intended placement and use of a dental material affect the properties that will be desired from the material. For some dental applications, a softer and more flexible dental material is desired, and for others a harder and less flexible dental material is desired. Thus, it is important to be able to cure a polymeric dental material into a finished form with physical qualities suited for the specific function that the dental material will perform in a given patient.
In general, short polymer chains can be very hard, but they lack flexibility. Long polymer chains tend to be more flexible, but it takes them longer to form. As the longer chains form, the dental material tends to move around resulting in gaps between a tooth on which the dental materials are placed and the dental materials. The length of the polymer chains is related to how long free radical complexes can link monomers together before encountering another free radical complex. When two free radical complexes encounter each other, a bond is formed between them and the polymer-forming reaction is terminated. If this happens quickly, short chains are formed. If this happens slowly, long chains are formed. The chemical reactions being described for polymer formation are photo-initiated free radical chain reactions. A photo-initiated free radical is formed when it absorbs light energy of the appropriate wavelength. If a high amount of light energy is applied to the material, then a high number of free radicals will be formed, they will encounter and contact each other often and consequently short polymer chains are formed. If a low amount of light energy is applied to the material, few radicals will form and long polymer chains will result. The speed at which radicals in these reactions encounter each other is algebraically proportional to the concentration of the free radical formed and is described in physical chemical kinetics as the xe2x80x9cTermination Ratexe2x80x9d. A specific example of this is diketone photo-initiated free radical polymerization which exhibits second order chemical kinetics. This means that the Termination Rate is directly proportional to the square of the concentration of free radical initially formed by absorbing light of the appropriate wavelength. Because of this algebraic relationship small changes in the amount of light being applied can produce significant changes in how many short chain, medium length chains and long chains form as the material is curing. The specific mixture or ratio of these chain lengths govern the post-cure physical properties. By selecting the amount of light energy applied to the material, the length of chains to be formed can be selected. By adjusting or modulating the amount of light energy applied to a polymeric material at selected times during the polymerization reaction, both the length of the polymer chains and the concentration of long chains to short chains in the resulting material may be controlled. This allows the precise physical properties of the resulting material to be achieved by controlling the polymerization reaction.
There are many variables at work which influence the properties of a material after the polymerization reaction is complete. These include the polymeric material that is started with, the nature of the light, the power of the light, the duration that the light is applied to the polymeric material, modulation of the light, and others. Control of the variables is important in order to achieve the desired result.
Some of the issues that should be considered when working with polymeric dental materials include shrinkage, flexural strength and surface or wear strength. Combinations of properties addressing these issues are desired when creating pit and fissure sealants, bonding agents, and orthodontic adhesives.
For the purposes of this document, the reader should be familiar with some terms. As used herein, xe2x80x9cpolymericxe2x80x9d means any material that cures by converting monomers and/or their derivatives (i.e. olgimers) into polymers. Restorative dental compound means a polymeric material that is used as a direct restorative material to fill cavities or rebuild teeth. Dental material means any polymeric material that is used in dentistry. Composite means a mixture that includes monomers and/or their derivatives (i.e. oligomers), and may also include dyes, filler materials, photo-initiator(s) and solvent(s). Cure means applying the appropriate wavelength of light to accelerate an initiator into a free radical state which, in turn converts monomers and/or their derivatives (i.e. oligomers) into polymers. Modulate means to vary intensity or power over time, such as in on/off, high/low, increase/decrease, combinations of these and other power adjustments. Dental surface means any surface of a tooth or dental appliance. This includes cutting surfaces (incisal), chewing surfaces (occlusal), vertical surfaces facing outward toward the face (facial), vertical surfaces facing inward toward the tongue (lingual), vertical surfaces facing toward the front of the head (mesial), vertical surfaces facing toward the back of the head (distal). xe2x80x9cPoly Chromatic Acusto Optic Modulatorxe2x80x9d means a device that by receiving either various input frequencies or acoustical waves separates and/or mixes and/or eliminates different wavelengths of light from a multiple wavelength light source. xe2x80x9cCurrent Frequencyxe2x80x9d or xe2x80x9cFrequencyxe2x80x9d means the number of times per second the electrical current charge for a positive (+) charge to a negative (xe2x88x92) charge (i.e, the frequency of 60 hertz changes from positive to negative 60 times per second).
B. Direct Restorative Dental Composites
A major concern in restorative dentistry is the shrinkage of a resin when it is cured. Once the cavity has been prepared, the composite is placed at or in the location of the tooth where tooth material had been removed and needs to be replaced. The composite is then cured. If the resin shrinks as it is cured, it will pull away from the tooth surface leaving a gap between the tooth and the resin. The gap provides a space where bacteria can leak past the restoration and cause an infection.
When curing dental material used to repair a cavity, by first applying a rapid influx of light energy (a high energy level), a matrix of short chain polymers may be set in the composite dental material very quickly. This matrix or xe2x80x9csetxe2x80x9d provides a rigid structure within the composite that reduces shrinkage. Once the short chain polymer matrix is formed, the amplitude of light energy can be reduced to a much lower level and held constant or otherwise adjusted. The lower level of light energy permits the remaining polymers to form long chains which can be used to provide flexibility or flexural strength in the polymerized dental material. Referring to FIG. 1, a graph is provided which illustrates one example of light power modulation with the intention of controlling or minimizing shrinkage of the dental material. The graph shows that initially light power is applied at a high level at 101. The particular high level used in the example is 250 miiliwatts. In the example, immediately upon application of the high power level it is continuously decreased to a desired point and then kept constant over time. The period of decrease in the example is about 3.5 seconds. Then the power is stabilized at a lower level 102, such as 100 milliwatts. The lower level in the example is 40% (or less than half) of the initial power level. Power is then maintained at a constant level 103, such as 100 milliwatts, for an additional period of time (6.5 seconds or more in the example). Note that this is approximately twice the time period of the power decrease period in the example, but could be any appropriate time period. The precise amount of time that light at high power is applied to the dental material, the way the power is reduced, the precise high and lower power levels, and the time that power is applied to the dental material are dependent both on the structural characteristics that are desired in the resulting dental material and are dependent on the composition of the dental material. In various dental materials, type and concentration of monomer, initiator, fill material and dyes vary and may require variation from the precise numbers and curve depicted in FIG. 1.
In certain restorations, in particular those that are on the chewing surfaces (occlusal surfaces) of the teeth, flexural strength is the post cure physical property of the dental material of most concern. For this application it is preferred that the material form the longest polymer chains possible in order to maximize flexibility. Referring to FIG. 2, a graph is depicted which indicates how it is preferred to modulate light power over time in order to maximize flexural strength of the dental material. As depicted in the graph, light power is initially kept at a constant level 201 (such as 100 milliwatts) for a period of time (such as 4 seconds) and then progressively increased over time 202. Note that in the example, the ending power level is about 2.5 times the initial constant power level. The precise amount of time that light is maintained at the constant power level, the way (rapidly, slowly or variably) the power is increased, the precise high and lower power levels, and the time that power is applied to the dental material are dependent both on the structural characteristics that are desired in the resulting dental material and are dependent on the composition of the dental material. In various dental materials, type and concentration of monomer, initiator, fill material and dyes vary and may require variation from the precise numbers and curve depicted in FIG. 2.
In some dental applications such as cutting surfaces (incisal surfaces), surface or wear strength of the material is of primary concern. In such an instance, modulation of light generally like that depicted in FIG. 3 tends to maximize surface or wear strength. That includes application of instantaneous high power 301 to produce strong short polymer chains on the surface of the dental material, reducing power over time to a low light source power level 302 which causes long chains to form deep in the dental material, followed by increasing the light power level over time 303 and ending at a high power level in order to finish polymerizing the dental material. The precise amount of power at various times during the curing process, the precise amount of curing time, the exact way the power is decreased and increased, the precise high and lower power levels, and the time that power is applied to the dental material are dependent both on the structural characteristics that are desired in the resulting dental material and are dependent on the composition of the dental material. In various dental materials, type and concentration of monomer, initiator, fill material and dyes vary and may require variation from the precise numbers and curve depicted in FIG. 3.
C. Pit and Fissure Sealants
Pit and fissure sealants require strong surface or wear strength but also must shrink around the tooth for a tight fit. Referring to FIG. 4, a graph is provided which depicts light source power modulation in order to provide optimum polymerization of a dental material for use as a pit or fissure sealant. Initially light power is applied at a high level 401 and increased some over time 402, then rapidly dropped 403 to a lower level 404, increased slightly and then kept constant 405 over time until the dental material is cured or fully polymerized. The precise amount of power at various times during the curing process, the precise amount of curing time, the exact way the power is modulated, the precise power levels, and the time that power is applied to the dental material are dependent both on the structural characteristics that are desired in the resulting dental material and are dependent on the composition of the dental material. In various dental materials, type and concentration of monomer, initiator, fill material and dyes vary and may require variation from the precise numbers and curve depicted in FIG. 4.
D. Bonding Agents for Indirect Applications
Bonding agents for indirect applications, such as crowns, bridges and veneers require maximum adhesion and flexibility, but face the unique problem that light energy must pass through the indirect restoration (crown, bridge, etc.) in order to reach the dental material to be polymerized. Referring to FIG. 5, a graph is depicted which is an example of light power source modulation requisite to penetrate the indirect restoration to start a slow polymerization reaction 501 (e.g., 100 milliwatts), increased according to a sine function to a high power level 502 (such as 250 milliwatts, and decreased 503 according to a sine function to a low power level 504 (such as 50 milliwatts), which allows the reaction to proceed and build long polymer chains. The light source power is then increased and decreased periodically according to a sine function and the cycle may be repeated more than once but perhaps many times in order to fully polymerize the dental material (i.e., in order to convert most or the majority of monomers in the dental material to polymers). The precise amount of power at various times during the curing process, the precise amount of curing time, the exact way the power is modulated, the precise power levels, and the time that power is applied to the dental material are dependent both on the structural characteristics that are desired in the resulting dental material and are dependent on the composition of the dental material. In various dental materials, type and concentration of monomer, initiator, fill material and dyes vary and may require variation from the precise numbers and curve depicted in FIG. 5.
E. Bonding Agents for Other Applications
Bonding agents for other applications, such as orthodontic applications, must strongly but temporarily affix orthodontic brackets to the enamel of teeth. The dental material used for this purpose should be strong enough to withstand the rigors of orthodontic treatment but brittle enough so that when the orthodontic treatment is concluded, the dental material may be shattered or broken in order to remove the orthodontic bracket without removing enamel from the tooth. Referring to FIG. 6, a graph is depicted which shows an example of light source power modulation in order to cure a polymeric dental material to have physical characteristics desirable for bonding agents for direct applications such as orthodontic brackets. As depicted in the graph, polymerization or cure is begun 601 with the light source at a low power for a period of time (1 second in the example). This initiates long chain polymer growth. The light source is then terminated. In the example, the light source is terminated for a time 602. Then light is reinitiated at a high power level 602 for a brief period of time (1 second in the example) in order to cause the desired brittleness in the dental material. The discontinuous nature of the power modulation curve is believed to work best for curing dental materials for orthodontic applications.
The precise amount of power at various times during the curing process, the precise amount of curing time, the exact way the power is modulated, the precise power levels, and the total time that power is applied to the dental material are dependent both on the structural characteristics that are desired in the resulting dental material and are dependent on the composition of the dental material. In various dental materials, type and concentration of monomer, initiator, fill material and dyes vary and may require variation from the precise numbers and light source application depicted in FIG. 6.
F. Types of Dental Materials
The preferred dental material used in the various embodiments of this invention and variations of those embodiments is a polymeric dental material that includes monomer(s) (which may be of various concentrations and type), initiator(s), fill material, dyes and solvent(s). Such a dental material is polymerized by exposing it to a light source of a wavelength that causes the initiator to start and carry out polymerization of the monomers into polymers of desired lengths. The light source must be matched with the initiator so that the light source is of a wavelength that initiates and carries out the polymerization reaction.
For reference. Table I below shows various initiators (initiators) and the wavelength of light to which each is sensitive. Any of these or other initiators may be used in the invention.
Also for reference, in Table II below a list of various commercially-available dental materials and their source is provided.
G. Methods for Modulating the Light Source
One preferred light source for use in the invention is a monochromatic laser of a wavelength matched to the dental material. Such an arrangement limits the dental material to a single initiator that absorbs light at the wavelength produced by the laser. Multiple initiators adapted for different wavelengths may be included in the dental material and multiple light sources of appropriate wavelengths for the initiators may be employed. It is preferred to use a computer controlled laser so that the exact waveform, modulation of the wave forms and power levels of light source can be produced consistently and accurately in order to achieve the desired post-cure physical properties from the composite being cured. The computer control can control the supply of electrical current to the laser and generate a variety of frequencies in different waveforms, so that as the electrical current is increased, the power output of the laser will increase, and as the electrical current is decreased the power output of the laser will correspondingly decrease mimicking the wave form and frequency generated. A light control circuit on the output side of the laser can provide exact measurement of the laser output power and feedback to the microprocessor, thereby allowing the microprocessor to deliver the pre-programmed desired power over time.
One preferred monochromatic laser for use with a single initiator (specifically camphorquinone) dental material is a 488 nanometer laser. An argon laser can be built such that it produces a very narrow band width of light around the 488 nanometer wavelength such that all photons are utilized by the initiator. The output power is monitored and adjusted according to light source power modulation techniques described herein.
The use of a single initiator in a dental material has become the standard in dentistry. The problem with single initiator dental materials is that they limit the post-cure physical properties of the dental material. The use of multiple initiators in the dental material permits the post-cure characteristics of the dental material to be more closely tailored to the desired application and is therefore preferred. Use of multiple light sources or the ability to change wavelengths one or more times during cure is necessary in order to take advantage of the presence of multiple initiators in the dental materials. Such examples would include but not be limited to multi-wavelength lasers (i.e. Krypton Ion Argon Ion mix) and mixed combinations of different lasers (i.e. and Argon Ion laser combined with an infrared diode laser in the same housing). The wavelengths of the multi-wavelength laser can be separated utilizing filters, prisms, diffraction gratings and/or Poly Chromatic Acusto Optic Modulators (also known as Acoust-Optic Tunable Filters). The preferred method would be with the Poly Chromatic Acusto Optic Modulator because it is capable of not only separating the individual wavelengths but recombining them in any percentage desired and the device is operated by applying current directly to it rather than having an electro-mechanical interface. An appropriate Poly Chromatic Acusto Optic Modulator can be obtained from Neos Technology, Inc., 4300-C Fortune Place, Melbourne Fla. However, any of the invented methods could be controlled (in the case of the other option listed by way of electro-mechanical interface such as servos and solenoids) with the same computer that controls the intensity modulation. Modulation of the intensity of the various wavelengths according to the modulation schemes described herein is also advantageous.
Although the most preferred light source used in this invention is a laser, conventional light sources (light sources other than lasers) may also be used. With non-laser light sources, however, it is difficult to produce monochromatic light. As conventional light sources produce light across a broad portion of the spectrum, it is typical to use a filter to limit the light emitted to the desired wavelength. The problem is that of producing sufficient intensities of and controlling the power of the specific wavelengths needed when there is no baseline of the percentage of power that is at the needed wavelength.
As a solution to this problem, the wavelength may be optimized by modulating input current (i.e., change the frequency of the current). When a conventional filament or short arc light bulb receives a particular frequency (i.e. 60 hertz as supplied in the US) it will produce a spectrum of light that may be more intense in the red and infrared portion of the spectrum whereas if the frequency is changed or the current is pulsed the light will produce a spectrum that is more intense in the blue wavelengths. With this understanding the utility of modifying the input current of a lamp used in curing becomes clear. Applying the current frequency that produces the greatest intensity of light in the wavelength that is needed in curing process and adding filters, prisms, diffraction gratings or other wavelength separating/eliminating devices would produce the perfect curing wavelength in much greater intensity than would otherwise be possible. It is then possible to provide an excess of the desired wavelength which can then be modulated over time to produce the post-cure physical properties desired from the material.
As an example of a dental material with more than one initiator, a two initiator dental material may be used that has a first initiator active in ultraviolet light and a second initiator active in the visible blue. The primary current to the light bulb would be modulated so that the ideal frequency for maximum ultraviolet output is achieved, the appropriate filters, prisms, diffraction gratings etc. would simultaneously be integrated into the system by the computer to eliminate the unwanted wavelengths. The microprocessor would monitor the output as described herein. The current amplitude may be increased or decreased to control the output power of ultraviolet light required. At the prescribed time the frequency of the power input would be adjusted by the microprocessor to achieve the greatest output of visible blue light, simultaneously the filter or prism or diffraction grating etc. would be changed by the computer to emit only the blue light. The amplitude of visible blue light would be measured and adjusted as already described. The use of changing the current frequency combined with conventional filtering methods provides wavelength control while simultaneously increasing/decreasing the current amplitude to modulate the intensity of the desired wavelengths produce curing control previously unavailable.
H. Examples of Light Source Power Modulation
Using the general inventive concepts outlined above, tests were performed in order to evaluate various modulation scenarios and their effectiveness for curing dental materials. The tests were designed to evaluate the effect that modulating curing light intensity over time has on the physical properties of a dental restorative material. Similar results will be expected for all areas of light-activated polymerization reaction, whether the intended field is dentistry or otherwise. In the tests performed, the restorative material was Herculite XRV available from Kerr Corporation in Orange, Calif., lot number 704675, expiration date 01/2000. The samples of that dental restorative material were prepared according to ANSI/ADA Specification Number 27 (1977). Six samples were used in each test. The samples were exposed to a particular light modulation scenario utilizing an argon laser (488 nm). The diametral tensile strength was measured in accordance with ANSI/ADA Specification 27 (1977) and the mean diametral tensile strength and standard deviation were calculated. The flexibility was assessed qualitatively and confirmed by a review of the diameters tensile strength and standard deviation. In interpreting the results, the reader should be aware that more flexible samples have a lower diametral tensile strength and the standard deviation is large due to flexing before breaking.
Test #1
Referring to FIG. 7, it can be seen that the samples were exposed to an initially low level (50 milliwatts) of light held constant for less than a second, and the light intensity was then increased steadily over a 10 second exposure time until the curing was completed in 10 seconds at a high power level of 250 milliwatts. The results of the test were as shown in the table below. The abbreviation xe2x80x9cMpaxe2x80x9d means megapascals.
Test #2
Referring to FIG. 8, it can be seen that the samples were exposed to an initially mid-level (100 milliwatts) of light held constant for five seconds, or half of the curing time. Then light intensity was increased over a 2 second period from 100 milliwatts to 250 milliwatts, and held constant at the higher power level until curing was complete. The results of the test were as shown in the table below.
Test #3
Referring to FIG. 9, it can be seen that the samples were exposed to an initially high level (250 milliwatts) of light held constant for 2 seconds. Then light intensity was abruptly decreased to a mid-level (100 milliwatts) from which it was gradually increased again to a high level (250 milliwatts) over the 10 second curing time. The results of the test were as shown in the table below.
Test #4
Referring to FIG. 10, it can be seen that the samples were exposed to an initially high level (250 milliwatts) of light held constant for 1 seconds. Then light intensity was incrementally stepped down about 50 milliwatts per step over 5 steps to a mid-level (100 milliwatts). At each step, the light was held constant for a brief period (about 0.5 seconds). The downward steps of light intensity were rapid but not instantaneous, as the graph shows. After 5 seconds of this gradual stepped modulation, the light level was held constant at a mid-level for the remainder of the planned curing time (in this case for an additional 5 seconds). The results of the test were as shown in the table below.
Test #5
Referring to FIG. 11, it can be seen that the samples were exposed to an initially high level (250 milliwatts) of light held constant for a brief time (about 0.5 seconds). Then light intensity was gradually but continuously decreased over the remainder fo the 10 second curing time, ending at a low power level (about 50 milliwatts). The results of the test were as shown in the table below.
Test #6
Referring to FIG. 12, it can be seen that the samples were exposed to an initial low level of light (50 milliwatts) which was rapidly increased to a high level (250 milliwatts) and decreased again to a low level (50 milliwatts) over a short time period (about 1 second). This modulation was repeated once per second over the 10 second cure time. The results of the test were as shown in the table below.
Test #7
Referring to FIG. 13, it can be seen that the samples were exposed to a constant moderately high level (200 milliwatts) of light for the entire 10 second curing time. The results of the test were as shown in the table below.
This type of curing is the current industry standard, and the post-cure properties of the dental material are typical of those achieved in the industry without use of the invention.
Summary of Test Results
Referring to FIG. 14, a graph is provided that compares diametral tensile strength, standard deviation and relative diametral shrinkage for each of the tests performed. Although for each test the same dental material was used as an input, the physical properties of cured dental materials from the different tests are significantly different from each other. Modulating the curing light power allows the physical properties of the resulting cured dental material to be designed and controlled. Additionally, the length of exposure of a dental material to a modulating light source will change its properties as well. Once the desired properties of the cured material for a particular application are known, power modulation and exposure times can be adjusted to produce a cured dental material meeting the desired criteria. By utilizing the invented modulation scenarios, the operator of a light source can easily decide which post-cure physical properties are desired for the particular dental application and then implement the modulation scheme which will achieve those post-cure properties. Alternatively, custom-cure modulation can be designed for unique applications.
I. Application to Various Dental Practice Scenarios
The scenarios below illustrate how a dental practitioner can implement the invention to achieve a superior cured dental material in his or her patients.
Scenario #1
If a dentist wishes to fill a large cavity on the occlusal (chewing) surface of a molar, the dentist would seek post-cure properties that include extreme hardness to avoid chipping on impact, flexibility to avoid the material fracturing under stress, and as little volumetric shrinkage as possible to avoid the potential for micro-leakage of bacteria into the tooth. For such an application, the practitioner would be best served by implementing the modulation scheme described for Test #4 above. A dental material cured by that modulation scheme exhibits excellent diametral tensile strength, good flexibility and only moderate shrinkage.
Scenario #2
If a dental practitioner wished to fill a small cavity at the junction where the tooth meets the gum line, he would want no shrinkage at all. Flexibility and hardness are not important in such a location because no force will be applied directly to the filling. Shrinkage, however, must be avoided so that the dentist will not need to perform any additional procedure to prevent micro-leakage. For such an application, the modulation scheme described for Text #6 above should be implemented.
Scenario #3
If a dental practitioner were applying orthodontic brackets, he or she would desire sufficient strength in the dental material to hold the brackets in place during orthodontic therapy, but it would be desirable to use a brittle dental material to permit shattering the dental material at the conclusion of orthodontic treatment without damage to the tooth or tooth enamel. At least 18 megapascals of tensile strength is needed for orthodontic applications. The practitioner should select either the modulation scheme of Test #3 or Test #7 for this application.
J. Industrial Applications
The invented modulation schemes and variations of them, while conceived of and tested for use in dentistry, have use wherever light-activated polymerization takes place. Below some examples of how the inventive concepts may be applied to other industries are listed.